Continuity mistake: In the scene where the Chamber finally opens, we first see Harry, Ron and Professor Lockhart from the front, and Harry and Ron are slightly behind Lockhart, and in a position to threaten and guard him. When you next see them a moment later from behind, Lockhart is behind them and they are in no position to guard him. (01:58:45)
Continuity mistake: In the girls' lavatory, Lockhart tries to bolt so Harry and Ron stop him and push him back. When Harry says, "You first," in this first shot his cloak not only drapes over the sink to his left, it's also under his hand's tight grip of the sink's wall. However in the next shot, still gripping the wall, his cloak now hangs behind him. Then after Ron says, "Better you than us," Lockhart's cloak is positioned the same way as the first shot. (01:59:20)
Continuity mistake: When Lockhart falls down the hole into the Chamber of Secrets, we hear him hit the ground a second or two later. When Ron and Harry jump down, not only do they take longer to get down, they also slide down the pipe, rather than fall straight down. (01:59:40)
Continuity mistake: When Gilderoy jumps into the entrance to the Chamber of Secrets you see his cape go flowing up above his head. In a short second shot, taken from above, it's not. (01:59:40)
Continuity mistake: The distance between Harry and Ron when they're sliding into the Chamber of Secrets keeps changing. Harry jumps first, and Ron jumps a few seconds later, so there should be a little distance between them. But when they're sliding down, Ron is directly behind Harry. At the bottom though, Harry slides onto the ground a few seconds before Ron, meaning they're again some distance apart. (02:00:10)
Continuity mistake: The smudges on Harry's face and the dirt on Harry's cloak, pants, etc., change in consecutive shots, throughout the shots after the rock slide, within the Chamber of Secrets and thereafter. (02:04:00)
Continuity mistake: When Harry just enters the Chamber he is running towards Ginny and drops his wand behind him, then Tom Riddle comes out and is to the front/side of Harry,he then picks up Harry's wand but he never walked behind Harry to pick it up, so the wand was in the wrong place. (02:04:45)
Continuity mistake: Tom Riddle says, "She won't wake" and walks toward Harry. When Harry says "Tom. Tom Riddle," the shot faces Harry and Ginny, and Harry's wand is lying on the open ground below the sole of Ginny's shoe. When Harry asks "Are you a ghost?" as the camera pans closer to Harry, his wand is conspicuously in a different place now, it is lying partially hidden under Ginny's robe above her right foot, right next to Harry. In the next shot after Riddle says, "A memory. Preserved...," the wand is gone from the previous shot's position. (Visible on DVD fullscreen.) (02:04:50)
Continuity mistake: When Harry says, "Tom. Tom Riddle," the shot faces Harry and Ginny, and Ginny's foot is parallel to the ground. In the next shot facing Riddle, Ginny's foot is perpendicular to the ground. When Harry asks, "Are you a ghost?" Ginny's foot is parallel. When Riddle says, "As poor Ginny grows weaker...," her foot is perpendicular to the ground once more and so forth. (02:04:50)
Continuity mistake: In the Chamber of Secrets and afterwards, the stain on Harry's collar is constantly changing. Some examples are the close-up as Harry leans over Ginny saying, "Ginny, oh please don't be dead. Wake up!..." and the close-up when Harry says, "Why do you care how I escaped? Voldemort was after your time." Also, in the close-up when Harry is crouching trapped with his back up against the grate, as the blinded Basilisk comes toward him, and then as he stands and looks at the Basilisk slithering away, up the pipe. (02:04:50)
Continuity mistake: When Tom Riddle takes Harry's wand in the Chamber of Secrets and Harry says "Give me my wand, Tom," his arm goes from being at his side to outstretched to back at his side in successive shots. (02:05:30)
Continuity mistake: In alternating shots while Harry is in the Chamber of Secrets, he has a pimple above his lip; in other shots he does not. (02:06:40 - 02:11:30)
Continuity mistake: When Harry is in the Chamber, running from the Basilisk, he trips and his glasses fly off his head, toward the left of the screen. In the wide shot as Harry trips, when his left hand touches the ground, the glasses are visible above his head. As his right hand touches the ground the glasses are again visible above his head. As the rest of his body touches ground, Harry's feet swing up in the air behind him and the glasses are now in the air way in front of him, just before they fly off to the left of the screen. However, in the next close-up the glasses are right in front of him and bounce toward his face. (02:10:00)
Continuity mistake: In the Chamber, when Harry runs from the Basilisk he ends up at a gated dead end, with steel bars that are embedded in a circular stone wall. When Harry crouches down, as the Basilisk's face nears, the manner in which the bars are embedded in the stone is actually quite different. (02:11:00)
Continuity mistake: The stripes on the knot of Harry's tie change position between certain shots, yet Harry does not redo his tie. For example, the diagonal red stripe is higher when he first speaks with Tom Riddle and lower when he kills Riddle and thanks Fawkes. (02:11:20 - 02:16:15)
Continuity mistake: When the Basilisk emerges from the water, in the first shot Harry looks at the Sorting Hat and Gryffindor's sword appears. As Harry gets the sword, in the second shot the Sorting Hat is not only positioned differently it is also lying on another part of the 'stone' floor. (02:12:30)
Continuity mistake: When Harry is in the Chamber of Secrets and he is fighting the basilisk, he drops the sword. The sword starts to fall and by the sound of it, it would have already have fallen off before he catches it. (02:13:40)
Continuity mistake: Harry pulls the sword out from the Basilisk's mouth, clutches at his right arm and the sword he holds has blood only at the tip. In the very next shot, as he clutches the tooth in his right arm, sword still in hand, there is now plenty of blood on the blade. (02:13:50)
Continuity mistake: In the scene where Harry puts the sword through the head of the snake you can see that only about half a foot of the sword is covered with blood. Then later in Dumbledore's office, when he asks Harry to take a closer look at the sword, you can see that there is blood over almost the entire sword. (02:13:50 - 02:20:50)
Continuity mistake: When the Basilisk falls with his face upside down and dies, there is not much blood at his mouth and on his teeth. However, as Harry passes the Basilisk on his way to Ginny, the snake's mouth and teeth are now covered with a substantial amount of blood. (02:14:10)
Answer: Yes. It's a common occurrence in a lot of movies for actors to forget their original lines so they improvise something to help move the scene along and not break character. Tom did this because he forgot what his original line was.